This invention relates to gold coatings and, in particular, to a method of determining the quality of a particular form of gold known as hard gold.
Hard gold is a particular form of gold widely used in the electronics industry for electrical contact finishes of improved wear resistance. Hard gold is typically defined as gold containing 0.1 to 0.3 percent by weight of a transition metal such as cobalt or nickel. As compared to pure gold it has greater hardness (120-200 KHV vs. 60-90 KHV for pure gold). More importantly, the resistance of hard gold deposits to sliding wear is greatly superior to that of pure gold deposits.
Hard gold is typically deposited by electroplating from mildly acidic solutions in which the source of gold is potassium gold cyanide and the transition metal is present as a salt or complex. It has been demonstrated that the transition metal is incorporated into the hard gold deposit in two different forms: a metallic form and a non-metallic form, often referred to as xe2x80x9cpolymerxe2x80x9d. The polymer has been identified as transition metal cyanide compound and has been correlated with the improved wear behavior of hard gold.
While high quality hard gold provides remarkable wear resistance, quality control of hard gold deposits has proved very difficult. The conventional approach has been to measure the total concentration of the transition metal or the hardness of the plating. Unfortunately, neither of these measures is a good indication of the performance a hard gold contact. Efforts to detect and quantify the polymer have not been successful as a method of quality control. Accordingly there is a need for an improved method of determining the quality of Hard Gold.
In accordance with the invention, the quality of hard gold is determined by Raman vibrational spectroscopy. A sample of the hard gold is provided, a monochromatic light beam is directed onto the sample, and the frequency and intensity of the light scattered by the sample is analyzed for the vibrational frequencies of polymer molecules. Scattering frequencies offset from the initial beam frequency by about 2132 cmxe2x88x921 and about 2182 cmxe2x88x921, for example, are indicative of high quality nickel-hardened gold.